1. Field of Invention
This invention is in the field of cross—talk suppression in a hybrid assembly at microwave frequencies.
2. Description of the Related Art
Monolithic Integrated Circuits (MMIC), an example of a semiconductor structure, support many of the present generation of military and commercial radio frequency sensors and communication applications. MMICs include active devices, such as field effect transistors and bipolar transistors, passive elements such as capacitors, thin film/bulk resistors, and inductors integrated on a single semi-insulating substrate, such as Gallium Arsenide.
Hybrid technology relates to methods used for interconnecting a plurality of separate semiconductor structures, such as MMICs, to a host substrate, in single, or multi-layer configurations. In a hybrid, inter-connections between the semiconductor structures is sometimes along the surface of the host substrate. These inter-connections are frequently made using metallized paths connected to bumps (soft solder, or hard plated bumps). These bumps, located on the surface of the substrate, engage conductive pads on the semiconductor structures thus forming conductive, interconnecting paths between the host substrate and the semiconductor structures. The bumps are used as a substitute in place of wire bonds for connections. The advantage of bumps over wire bonds include the elimination of wafer backside processing steps such as wafer thinning, via formation, and metal deposition.
Another advantage to using surface bumps for interconnection purposes is the lower thermal resistance between the semiconductor structures and the host substrate. The lower thermal resistance of the bump connection is due to the relatively large surface area of contact between the host substrate and the semiconductor structures. Heat transfer is also facilitated by the large diameter and short length of the bump, as compared to a wire interconnect. Although both the bump and the wire are made of thermally conductive metal, the favorable aspect ratio of the bump and wider surface area present a lower thermal resistance as compared to a typically thin, long wire bond. The lower thermal path presented by the bump facilitates the conduction of heat away from the semiconductor structures, allowing higher power density for the semiconductor/substrate hybrid assembly, especially when using thermal bumps directly under heat sources. The higher power density allows higher performance for the hybrid.
Yet another advantage of using bumps for interconnect purposes is the elimination of parasitic effects such as capacitance, inductance and radio emissions present with wire bonds and vias. At high frequencies, the thin, long wire bonds, and the vias traversing the thickness of the substrate can be considered as antennas for the emission of electromagnetic interference. The same wires and vias present capacitance to adjacent structures, as well as an inductance to the signals transmitted by the wires.
Other advantages of bumps are their lower cost and higher reliability. Typically bump type connections can be efficiently completed using a single epoxy cure/solder reflow die-attach process. This presents fewer steps during manufacture as compared to wire bond techniques. With bump interconnect, there are no mechanical wire connections to shake loose, be intermittent or fail due to thermal cycling.
While bumps are advantageous as compared to wire inter-connections, their presence between a semiconductor structure and a host substrate presents unique electromagnetic resonance and emission packaging problems. First, there is the optimization of the vertical radio frequency interconnect transitions presented by the interface between the bumps on the host substrate and the semiconductor structure mounted thereon. Then there is the potential electromagnetic coupling effects presented at the interface between the semiconductor structure and the host substrate, as well as the host substrate opposing surfaces.
A particular difficulty introduced by the semiconductor structure mounted on the host substrate is the potential formation of electromagnetic boundaries which sup port unwanted, parallel plate, waveguide like (surface modes) of energy propagation. Such unwanted modes can propagate near the surface of the host substrate causing degradation in semiconductor performance because of signal interference. The degradation in semiconductor performance are caused by unwanted signal transfer among semiconductor structure inputs and outputs, affecting gain and phase response, loss of isolation between adjacent paths in multiple path/multiple channel circuit applications, and circuit instability. These negative effects are due to the introduction of unwanted coupling or feedback paths.
Maximum frequency operation of the semiconductor in the presence of these unwanted feedback paths are undesirably dependent on the dimensions of the semiconductor structure. Thus, semiconductor structures with large dimensions with respect to wavelength operating frequency present a potential difficulty. This difficulty is prevalent with fast Gallium Arsenide (GaAs) semiconductor structures mounted on a host substrate. The relatively large semiconductor size of GaAs as compared to the wavelength of the operating frequency approach the cutoff frequencies at the upper edge of the operational band. Near cutoff, the semiconductor structure may be functional, but unable to operate because the incoming signals are interfering with each other.
In the prior art, signal interference is reduced by incorporating grounded interconnect bumps strategically placed on the semiconductor structure surface to break up surface modes of energy propagation. By making direct contact to ground pads on the host substrate, the path of the surface modes is disrupted. These grounded interconnect bumps act like shields and attempt to change the path to be followed by the electromagnetic energy propagated along the surface of the host substrate. Unfortunately, because of semiconductor structure limitations and assembly requirements, this practice of using redundant ground bumps as obstacles to surface propagated electromagnetic waves results in a further increase in both semiconductor structure and substrate size, increasing weight, power consumption and reducing reliability of the resulting hybrid.